This invention relates to a veneer lathe for peeling a veneer from a log by holding the log in position and rotating it about its horizontal axis by means of spindles applied fast one each to the opposite end faces of the log and pressing a knife of a length enough at least to extend over the entire length of the log against the periphery of the log in rotation. More particularly, this invention relates to a veneer lathe capable of increasing the yield of a high-quality veneer usable as face and back veneers in plywood which determines the commercial value of the finished plywood and most suitable also for the production of a veneer from low-grade logs of embrittled or cracked texture.
Recently in the plywood manufacturing plants, logs of inferior quality have come to be used in consequence of worsening lumber situation or for the purpose of keeping the plywood production cost from rising by reducing lumber price which accounts for the major portion of the production cost.
In the meantime, with a view to improving the yield of veneer from logs used and enhancing the operational efficiency of the shop, there has arisen intensifying demand for a veneer lathe capable of producing a veneer of good quality from a log of poor quality at a high speed and until the log is peeled to the smallest possible diameter.
Generally, the veneer lathe, in producing a veneer from a log, necessitates use of a so-called pressure member for exerting pressure to the freshly peeled veneer against the log near the edge of a knife as an indispensable element. As the pressure member, the stationary pressure nose bar and the rotary roller bar (of both driver and follower types) have been known to the art. The stationary pressure nose bar has an advantage that it can be easily installed at a prescribed position derivable from the particular cutting conditions involved. It, on the other hand, has a disadvantage that a great load resistance is generated during the peeling of veneer and, therefore, the log being cut is liable to sustain breakage because the area of pressure application by the pressure nose bar extends throughout the entire length of the log. In contrast, the rotary roller bar offers very small resistance during the peeling of veneer. Particularly the roller bar of the type adapted to operate with driven rollers is allowed to decrease the resistance to a great extent. The roller bar, nevertheless, has a disadvantage that it is difficult for the roller bar to be accurately set at the prescribed position derivable from the particular thickness of veneer being peeled off the log.
In the production of a veneer by the operation of the veneer lathe, the best results are obtained when the pressure is applied at a position separated from the edge of the knife by a distance of about 20% of the thickness of the veneer desired to be produced in the direction opposite the direction of the rotation of the log. Satisfaction of this particular condition becomes increasingly more impracticable in proportion as the diameter of the roller bar increases. As the diameter of the roller bar increases and the periphery of the roller bar is consequently brought closer to the aforementioned prescribed position for pressure application to the log, the rear face of the knife and the lower portion of the periphery of the roller bar approach each other so much that the roller bar comes into contact with the rear face of the knife before it reaches the prescribed position for pressure application, with the result that desired application of pressure at the prescribed position will become completely impracticable. Thus, the aforementioned condition becomes severe in proportion as the thickness of the veneer desired to be peeled is decreased. As one solution for the difficulty, there has been established a practice of limiting the maximum diameter of the roller bar only to about 16 mm. Since the diameter of the roller bar is so small and the revolution number is inevitably increased to extremity, the burden on the bearing of the roller bar is so enormous as to call for a special lubricating system.
Further, as disclosed in U.S. Pat. No. 4,221,247 there has been lately developed a veneer lathe comprising pressure-contacting disks provided with a large number of saw-teethed blades on their periphery against a log and rotating the log by driving the disks.
The veneer lathe adapted to be peripherally driven as described above, however, has a possibility that the peripheral driving will tend to inflict scratches and cracks upon the log, the scratches and cracks will collect wood chips thereon when they are squeezed between the log surface and the pressure bar, and the chips will occasionally find their way past the pressure bar to impart flaws to the surface of the veneer, clog the knife opening for the pressure nose bar of the veneer lathe and compel the operation of the veneer lathe to be interrupted, impair the uniform distribution of the pressure of the pressure bar, and degrade the quality of the produced veneer. This veneer lathe has another disadvantage that the pressure used in driving the saw-teethed blades of the discs into the log will warp the log even to a point where it is difficult for the peeling of the veneer to be continued until a sufficiently small diameter of the log. Further, this veneer lathe leaves numerous punctures in the peeled veneer. Particularly when the veneer is produced from a brittle log or it is produced in a small thickness of less than 1 mm, such punctures possibly give rise to tears in the veneer while the veneer is peeled off the log or it is transported. The veneer containing such tears can no longer be used as outer veneers in a plywood which determine the commercial value of the produced plywood. Even when the veneer is used in intermediate veneers, the yield is still low.